int (0 +00) x^(2n) * e^(-x^2) dx = | t = x^2; x = t^(1/2); dx = 1/2 * t^(-1/2) dt | =
= int (0 +00) t^n * e^(-t) * 1/2 * t^(-1/2) dt = 1/2 * int (0 +00) t^(n - 1/2) * e^(-t) dt =
= 1/2 * Г(n + 1/2) = 1/2 * (2n - 1)!!/2^n * pi^(1/2) = (2n - 1)!!/2^(n + 1) * pi^(1/2).